Researchers at Georgia Tech and MIT have developed a way to automate the process of finding and recording information from neurons in the living brain. The researchers have shown that a robotic arm guided by a cell-detecting computer algorithm can identify and record from neurons in the living mouse brain with better accuracy and
speed than a human experimenter. Using this technique, scientists could classify the thousands of different types of cells in the brain, map how they connect to each other, and figure out how diseased cells differ from normal cells.

Graphene-based transistors that respond to changes in chemical solutions could be used to link electronic devices directly to the human nervous system. That is the claim of researchers in Germany who have built arrays of devices that respond to changes in the electrolytes surrounding living cells. The team hopes that its research could result in retinal implants that could help some visually impaired people see images.

The research focuses on the small voltage that a neuron creates across its cell membrane when it fires, with the potential difference arising from sodium ions moving into the cell and potassium ions moving out into the surrounding solution. Since the 1970s, biophysicists have been trying to detect this sudden change in the electrolytic properties of the liquid surrounding a cell using a type of field-effect transistor (FET). These devices are called Solution-Gated FETs (SGFETs) and much of the initial research was done using silicon. But after graphene was isolated in 2004, some researchers realized that this material – a layer of carbon just one atom thick – could be used to create better SGFETs.

According to Jose Garrido of the Technische Universität München, who has led the work, graphene offers several important advantages over silicon. First, the graphene surface remains clean – unlike silicon, which quickly forms a performance-degrading oxide layer when exposed to the electrolyte. Second, electrons in graphene have an extremely high mobility, which makes the device much more sensitive than silicon SGFETs. Finally, graphene is extremely flexible, which is good because any device implanted within the brain or similar tissue must be bendable.

A SGFET is a different take on a conventional graphene FET, in which the current flowing through its graphene channel can be controlled by changing the voltage applied to a nearby “gate” electrode. In a SGFET, in contrast, the gate voltage is kept constant and the graphene is exposed to the electrolytic environment of the cell. Any shift in the concentration of ions in the solution affects the electronic properties of the graphene thereby changing its conductivity and the current flowing in the graphene channel. The firing of a neuron is therefore detected as an electronic signal.

In their new work, Garrido and colleagues have created 8 × 8 arrays of SGFETs – with each individual transistor measuring about 10 μm across. These arrays were used to detect firing signals from neuron cells that were cultured on an artificial medium. The researchers have also shown that neuron cells are able to survive for long periods of time in close proximity to graphene layers. They now want to show that the SGFETs work in living tissue – rather than cell cultures – and that neuronal tissue is not adversely affected by the presence of the devices.

According to Garrido, an important application of the graphene SGFETs would be creating retinal implants that could improve the sight of visually impaired people. Indeed, he believes that an array containing about 1000 elements could provide the brain with enough information for a person to be able to perceive an image. Another important application could be as cortical implants to help people control artificial limbs.

Although creating a 1000-element array of graphene SGFETs is a straightforward process, Garrido says that integrating the technology within a person will require a great deal more work.

Graphene can function much like a laser when excited with very short femtosecond (fs) light pulses. The team of scientists has shown that the material has two technologically important properties – population inversion of electrons and optical gain. The findings suggest that graphene could be used to make a variety of optoelectronics devices, including broadband optical amplifiers, high-speed modulators, and absorbers for telecommunications and ultra-fast lasers.

Graphene is a sheet of carbon atoms arranged in a honeycomb-like lattice just one atom thick. Graphene has unique electronic and mechanical properties. It could find use in number of technological applications – even replacing silicon as the electronics industry’s material of choice in the future. This is because of the fact that electrons whizz through graphene at extremely high speeds, behaving like “Dirac” particles with no rest mass.

The material could also be an ideal candidate for photonics applications especially optical communications, where speed is all-important. For example, it has an ideal “internal quantum efficiency” because almost every photon absorbed by graphene generates an electron–hole pair that could, in principle, be converted into electric current. Thanks to its Dirac electrons, it can also absorb light of any colour and responds extremely fast to light, which suggests that it could be used to create devices much faster than any employed in optical telecommunications today.

Researchers have already shown that they can make basic devices, such as solar cells, light emitters, touch screens and photodetectors from graphene. However, few studies have looked at what happens when the material is excited with femtosecond (fs) light pulses that create so-called non-equilibrium charge states – particularly the state consisting of extremely dense Dirac electrons. Materials that harbour such states have nonlinear optical properties that are important for making real-world optical devices, such as ultrafast modulators, amplifiers and wavelength converters.

Epitaxially grown Graphene Monolayer with Pump Laser Pulses (t = 35 fs) and photon energy of around 1.55 eV has been demonstrated by researchers. The reflection of light from the samples have been measured because Graphene is just one atom thick and has zero energy electronic bandgap. This measurement provides information on the amount of light absorbed by the material. This in turn depends on the optical conductivity of graphene.

Researchers found that the optical conductivity changes from being positive to negative as the intensity of the pump pulses increases. This means that more light is coming out of the material than going in, something that indicates optical gain.

Researchers also demonstrated that the intense external pump laser pulses excite electrons in graphene so that more of these charge carriers exist in the upper “Dirac cone” – the conduction band of the material – than in the lower cone. Once such a population inversion has occurred, a probe photon then stimulates these excited states to emit infrared light in a coherent cascade. The coherent light emitted shows gain on the order of about 1%, a value that is much greater than those seen in conventional semiconductor optical amplifiers – a surprising result since graphene is merely one-atom thick.

Researchers have also found that this optical gain could be observed over a wide range of energies – up to hundreds of milli-electronvolts (meVs) below the pump photon energy. Such a broad optical gain might be unique to graphene and related to the fact that photoexcited electrons in the material scatter extremely fast among themselves. What is more, an ultrashort pulse just 35 fs long is sufficient to produce this broadband gain – something that has never been seen before in any material.

The population inversion and resulting optical gain in the infrared part of the electromagnetic spectrum confirms graphene’s potential for applications such as broadband optical amplifiers, lasers and in telecommunications. However, there is still much to do before this happens, says Wang, who is now looking at further characterizing the photoexcited graphene states in the near-infrared to the mid- and far-infrared spectral regions.

Dopamine (DA) is an important neurotransmitter, deficiency of which underlies Parkinson’s diseases. DA detection is challenged by its low physiological concentration (0.01 mM–1 mM) and interference from much more abundant ascorbic acid (AA) and uric acid (UA).

A chitosan–RGO composite electrode for DA detection was demonstrated in literature. A linear detection range (5–200 mM) was achieved in the presence of a large excess of AA or UA (500 mM). In addition, it showed that the chitosan–RGO electrode outperformed the electrode made of chitosan and multi-walled carbon nanotubes.

An electrochemical sensor was also demonstrated to selectively detect dopamine with a LOD of 0.01 mM based on a composite electrode made of Nafion and N-(trimethoxysilylpropyl) ethylenediamine triacetic acid (EDTA) modified RGO.

The high performance arises from several reasons:

(1) dopamine can interact with RGO via p–p interaction;

(2) EDTA groups, combined with ionic sulfuric groups of Nafion, can concentrate DA from the solution;

(3) EDTA groups linked to the RGO surface promote electron transfer as evidenced by the narrower potential separation between the anodic and cathodic peaks (DEp);

(4) the oxygen containing functional groups on RGO block the diffusion of AA and thus eliminate its interference.

In another work, detection of DA at 5 nM was realized in the presence of excess AA using a b-cyclodextrin/RGO nanocomposite electrode. b-Cyclodextrin functionalization assists dispersion of RGO sheets, and greatly improves the electrochemical performance.

As compared with the bare RGO electrodes, the b-cyclodextrin/RGO electrodes exhibited a two orders- of-magnitude-lower LOD, attributable, at least in part, to the faster electron transfer rate (DEp was reduced from 115 mV to 73 mV).

AA and UA sensors have also been developed using graphene materials. An AA sensor using graphene nano-sheets exfoliated in liquid by dimethylformamide (DMF). A UA sensor was constructed by self-assembling gold nanoparticles (AuNPs) onto pyrenebutyrate functionalized RGO (PFG) sheets. A LOD of 0.2 mM was obtained.

Novel microwave plasma enhanced CVD method was utilized to obtain multilayer graphene nanoflake films (MGNFs) vertically grown on a silicon substrate. DA, AA, and UA can be unambiguously distinguished by three well-defined peaks that appeared in the cyclic voltammogram (CV). Furthermore, near-ideal electron transfer kinetics was evidenced by the narrow DEp (61.5 mV at the scan rate of 10 mV s-1) which is close to the ideal value of 59 mV. Such a fast electron transfer process is due to the abundant edge planes and defects on the nanoflakes, unique electronic structure of graphene, and the good electrical contact between MGNFs and silicon substrate.

Cholesterol is an essential constituent of cell membranes. However, undesired accumulation of cholesterol and its esters causes critical health problems, such as heart diseases, cerebral thrombosis, and atherosclerosis.

A sensitive amperometric sensor based on functionalized RGO sheets has been developed for detection of cholesterol and its esters with a LOD of 0.2 mM.161 Cholesterol esterases and cholesterol oxidases were loaded onto the electrode to catalyze the hydrolysis of cholesterol and its esters, and consequently, generate H2O2. Platinum nanoparticles decorated on RGO sheets, in turn, catalyze the electrochemical oxidization of H2O2. Nafion coating was used at the same time to block other irrelevant analytes (e.g., ascorbate and urate).

Extracted and edited from “Biological and chemical sensors based on graphene materials by Yuxin Liu, Xiaochen Dong and Peng Chen in Chemical Society Reviews, 2012″

Detecting rare pathological cells is of obvious clinical significance. Sensitive and selective RGO-based electrochemical biosensors were developed and demonstrated in literature to detect cancer cells with over expressed nucleolin on plasma membrane (e.g. breast cancer cells and human cervical carcinoma cells), at a LOD of thousand cells per ml.

To avoid RGO aggregation and introduce more –COOH groups, 3,4,9,10-perylene tetracarboxylic acid (PTCA) was used as a composite with RGO. The nanocomposite was covalently functionalized with NH2-modified nucleolin-specific aptamers (oligonucleotides serving as highly selective antibodies) as the recognition element. The binding of cancer cells increases the electron transfer resistance by blocking the access of the redox probe ([Fe(CN)6] 3-/ 4-). Electrochemical detection in amperometry mode provides high temporal resolution (milliseconds). Therefore, it is suitable to detect dynamic cellular activities in real-time.

A RGO based sensor for detection of the real-time kinetics of oxygen release from human erythrocytes in response to NaNO2 stimulation has been shown. Two kinds of excellent mediators for O2 reduction, namely, laccase (Lac) and 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), were functionalized onto RGO sheets to form a Lac–ABTS–RGO hybrid electrode.

An O2 level as low as 10 mM can be detected by this hybrid electrode. Cellular release of reactive oxygen species (such as H2O2) is an early indicator for cytotoxic events and cellular disorders. A RGO based electrochemical sensor has been coupled with live human breast cancer cells (MCF-7) to detect triggered cellular release of H2O2 in real-time and with a LOD of 0.1 mM.

To construct the electrode, RGO sheets were first electro-phoretically deposited on the indium tin oxide (ITO) glass. This was followed by electro-deposition of Prussian blue (artificial H2O2 catalyst) and adsorption of extracellular matrix proteins (laminin) to promote cell adhesion. Ten layers of RGO–PB–laminin were formed on the ITO substrate using layer-by-layer deposition. In situ, real-time, sensitive, and quantitative detection of extracellular H2O2 release from live cells was demonstrated. Specifically, it was determined that, upon stimulation of phorbol-12-myristate-13-acetate (PMA, 5 mg ml-1), 1011 H2O2 molecules were released from a single MCF-7 cell over 25 s.

Extracted and edited from “Biological and chemical sensors based on graphene materials by Yuxin Liu, Xiaochen Dong and Peng Chen in Chemical Society Reviews, 2012″

Graphene based electrochemical sensors have also been developed to detect various protein biomarkers. A label-free immuno-sensor to specifically detect cancer marker alpha fetoprotein (AFP) using layer-by-layer construction with Electropolymerized Thionine (TH) film, GO–chitosan composite, AuNPs, and conjugates of horseradish peroxidase (HRP) and anti-AFP antibody has been fabricated and reported in the literature.

Binding of AFP molecules to the antibodies partially blocks the active center of HRP and consequently decreases the catalytic reduction of H2O2 by HRP (thus a decrease in the electrochemical signal). The electroactive TH acts synergistically with HRP to mediate the electron transfer from H2O2 to the electrode. The achieved LOD (0.7 ng ml-1) is much better than the conventional Enzyme-Linked Immuno-Sorbent Assays (ELISA). This sensor was challenged with clinical human serum samples and the negative/positive samples were correctly identified in accordance with the results from a commercial clinical device.

A simpler AFP sensor was made by incorporating TH with RGO film through p–p interaction followed by covalent cross linking of AFP antibodies with TH. Binding of AFP molecules blocks the electron-transfer and mass-transfer, leading to a decrease of electrochemical signal originated from the redox reactions of TH.

In comparison with other sensors, such as carbon nanotube or nanoparticle derived AFP sensors, a much lower LOD (5.77 pg ml-1) was achieved, due to the high electron transfer rate between the intimately interacted RGO and TH, and high loading of TH molecules and AFP-antibodies because of the large surface area provided by the RGO film. The sensor was successfully used to determine AFP in serum samples.

A different strategy to detect AFP was also demonstrated. AFP molecules bound to the primary-antibody functionalized RGO electrode complex again with carbon nanospheres (CNS) tagged with the secondary antibodies and HRP molecules, leading to an increased electrochemical signal from redox reaction of H2O2. The use of RGO and CNS gave a 7-fold increase in the detection sensitivity, because of the superior electrochemical and electrical properties of RGO and the ability of CNS to carry multiple HRP molecules.

A 20 pg ml-1 LOD was demonstrated. A similar sensor to detect prostate-specific antigen (PSA) (marker for prostate cancer) based on sandwich immuno-reactions on top of RGO modified electrode has been reported. In comparison, the CNS were replaced by small RGO flakes, because RGO flakes can carry more secondary antibodies and more HRP molecules due to their extremely large surface-to volume ratio. Dual functionalities of RGO were utilized, i.e., first as the electrode material and second as the enzyme carrier. An impressive detection limit of 1 pg ml-1 was demonstrated, superior to other PSA sensors including a sensor using carbon nanotube–HRP conjugates.

A sandwich-like immuno-detection of carcino-embryonic antigen (CEA) which is a marker for colorectal cancer was developed and reported. In their work, a nanocomposite of gold nanoparticles (AuNPs), RGO and chitosan was used to carry multi-copies of the HRP-conjugated CEA-specific secondary antibody onto a glassy carbon electrode modified with Prussian blue and AuNP. 10 pg ml-1 CEA can be detected.

In another demonstration, a RGO modified electrode for sandwich-like immuno-detection of immuno-globulin G (IgG) in human serum was developed.

Graphene materials have also been employed for sensitive and selective electrochemical detection of nucleobases, nucleotides, single stranded DNAs (ssDNA), and double stranded DNAs (dsDNA). Such electrochemical DNA sensors may provide a simple alternative approach for DNA analysis and sequencing.

The four distinct nucleobases (A: adenine, T: thymine, C:cytosine, G: guanine) can be electrochemically differentiated because they have different oxidation potentials. Demonstrations using RGO with abundant –COOH groups to electrochemically detect guanine and adenine with a LOD of 50 nM and 25 nM, respectively has been reported in the literature. The high sensitivity can be ascribed to the excellent electrochemical properties of RGO, the electrostatic attraction between the negatively charged –COOH groups and the positively charged nucleobases, and the strong p–p stacking interaction between the nucleobases and honeycomb carbon lattice.

A Fe3O4 nanoparticle doped RGO–chitosan electrode has been used to detect guanosine. It was suggested that Fe3O4 nanoparticles help to reduce the electron transfer resistance.

RGO based electrode decorated with AuNPs by potentiostatic electro deposition to detect ssDNA has been shown in the literature. The incorporation of AuNPs was proven to be essential to separate the oxidation signal of T from that of A. It was also demonstrated that the electrochemically reduced RGO showed enhanced electrochemical and electro catalytic activity as compared to chemically reduced RGO. This DNA sensor is able to detect single-base alteration (mutation) without any labeling or probe DNA.

Stacked graphene nanofibers (SGNFs) were used to distinguish the four nucleobases with a sensitivity two to four folds higher than carbon nanotube-based electrodes. The high sensitivity is due to numerous open edges of individual graphene nanosheets which are much more electrochemically active compared to the basal carbon plane. This sensor was employed to examine the base composition of human influenza A(H1N1) DNA strand.

Graphene epitaxially grown on SiC was used to detect dsDNA was also demonstrated in the literature. It was shown that dsDNA can be differentiated from ssDNA, because dsDNA exhibits lower oxidation peaks for A and C and increased oxidation potential for C. Electrochemical detection of dsDNA is not possible with the conventional electrodes (e.g., gold electrode and GCE) due to their limited electrochemical potential window. It was also shown that electrochemical anodization to introduce oxygenated groups onto graphene largely improved the electrode performance.

A GO modified electrode was used for detection of DNA hybridization. In this work, probe ssDNA molecules that lack guanine base were covalently immobilized onto a GO film, and hybridization was detected by the guanine oxidation signal from the target ssDNA molecules (a hepatitis B virus specific sequence).

An interesting demonstration showed RGO quantum dots (B10 nm) used to modify the pyrolytic graphite electrode for detection of DNA hybridization. When the target ssDNA hybridizes with the pre-immobilized probe ssDNA, the electron transfer from the electrochemically active species [Fe(CN)6]3-/4- was increased because the blocking effect by the probe ssDNA was alleviated. A LOD of 100 nM was reached. This study suggests the potentials of RGO quantum dots in electrochemical sensing. The good performance of RGO quantum dots may be attributed to their abundant edge sites (electrochemically active sites) and quantum confinement effects.

Based on a similar sensing scheme, a RGO based sensor to detect hybridization of methicillin-resistant Staphylococcus aureus DNA with a LOD of 100 fM was shown. However, the scientists proposed an opposite mechanism. They argued that hybridized DNAs remained on the RGO surface and caused an increase of electron transfer resistance (hence a decrease in the electrochemical signal). The discrepancy between both works may be because of the size difference between RGO quantum dots and RGO sheets. Larger RGO sheets likely can bind more strongly with hybridized DNAs.

Hypoxanthine is a purine derivative. A hypoxanthine sensor was constructed using an electrode consisting of RGO, conducting polypyrrole graft copolymer, poly(styrenesulfonic acid-g-pyrrole), and enzyme xanthine oxidase. The detection mechanism of such a sensor involved two-steps of oxidation: oxidation of hypoxanthine catalysed by xanthine oxidase, and subsequent oxidation of uric acid and H2O2 produced from the previous reaction. RGO and the conducting polymer interact with p–p stacking and form a nanocomposite with high conductivity and an excellent electrocatalytic environment. As a result, a LOD of 10 nM was obtained. As hypoxanthine accumulates continuously from adenine nucleotide degradation after fish death, this sensor was employed to assess fish freshness.